1. Field of the Invention
The present invention generally relates to a highly brightness enhancing optical element for a backlight type LCD, and more particularly, to a highly brightness enhancing optical element integrated with a polarizer for a LCD with backlight sub-system supporting.
2. Description of the Prior Art
In all kinds of flat-panel displays, liquid crystal display (LCD) is the only one that employs linearly polarized light to produce brightness, darkness, and grey scale. A linearly polarized light is generated by passing light from backlight module through a polarizer. The linearly polarized light is then shooting into liquid crystal cell to produce brightness and darkness according to the arrangement of liquid crystal molecules in the cell.
However, the final output of the light is only 4-6% of that provided by backlight source. One of the major losses is from the dichroic polarizers of LCD that absorbs half of incident light. Therefore, if the incident light can be transformed to one of the linearly polarized light that is able to totally transmit the polarizer, the efficiency of the backlight system can be significantly improved and the brightness of the current LCD can thus be enhanced.
A reflective type polarizer has been developed as a brightness enhancement optical element for LCD, which does not absorb light itself. One of the reflective type polarizer is a cholesteric liquid crystal reflective type polarizer, which can turn an unpolarized white light into a circularly polarized lights, left-handness circularly polarized light or right-handness circularly polarized light. One of the circularly polarized lights will transmit the cholesteric liquid crystal film, while the other will be reflected. The circularly polarized light originally reflected by the cholesteric liquid crystal film can be easily converted to a transmissible circularly polarized light by using a simple surface of reflective character. The backlight module of a LCD usually contains such a reflective mechanism. The transmissible circularly polarized light will pass through the cholesteric liquid crystal film. This means all the light from backlight system will, in theoretically, all pass the polarizer and thus generates right or left-handness circularly polarized light with double light intensity. The incident unpolarized white light from the backlight module can be converted to a linearly polarized light with double light intensity. According to optics fact, if a quarter-wave plate is attached to the cholesteric liquid crystal film. Such a brightness enhancement optical element can be seen in U.S. Pat. No. 5,506,704, for example.
U.S. Pat. No. 5,506,704 discloses a reflective polarizer made by assembling a quarter-wave plate and a polarizer to a cholesteric liquid crystal, wherein the quarter-wave plate is formed by solvent casting or extruding and then being precisely stretched. To assembly the reflective polarizer, the quarter-wave plate needs to be coated with an optical paste to make sure that the quarter-wave plate is successfully adhered to the cholesteric liquid crystal. To assure integrity of the quarter-wave plate before it is employed, it is covered with a protective film. Accordingly, the protective film needs to be stripped before proceeding with the adhering step, thereby not only wasting additional material and increasing complexity of manufacturing but increasing the number of interfaces between several layers, which results in optical transmission loss. The polarizer is generally made by a more complex extension process, which has an upper and a bottom TAC film and an optical paste layer. As a result, the polarizer has more interfaces and is thicker in comparison with the quarter-wave plate, which increases the cost of manufacturing and cause a reduction in transmittance, and thus adversely affects the performance of the optical element.
U.S. Pat. Nos. 5,601,884 and 5,743,980 disclose a method of preparing a phase delay plate by coating on a glass substrate with a birefringent liquid crystal. U.S. Pat. No. 6,262,788 B1 discloses a method of preparing a phase delay plate by coating a liquid crystal on TAC film. The objectives of the aforesaid US patents are not directed to an optical conversion for segregated lights from the cholesteric liquid crystal film. In regard to the fabrication of a polarizer, U.S. Pat. No. 6,049,428 discloses a novel E-type polarizer having a transmitting axis parallel to the molecular optical axis, which is different from the conventional O-type polarizer having a transmitting axis perpendicular to the molecular optical axis. However the E-type polarizer is basically a light absorption type polarizer, and doesn't have mechanisms for converting a light polarized state and recovering light to enhance brightness.
As to the preparation of the polarizer, conventionally, the polarizer is prepared by stretching the PVA (polyvinyl alcohol) film in iodinate containing Solution so as to grow microcrystalline and is then covered with a TAC (Triacetate Cellulose) film. However, the iodinate is a poison material and the PVA stretching processes to grown crystalline are rather complicated and lousy. Apart from that, the stretched PVA type polarizer is found to absorb more than 50% of light. As a result, the transmittance is 38-44% only in reality. Thus the conventional polarizer is a high energy consumption product.